In one scavenging air valve of this genre that has been known for years, the valve body is structured as a rubber membrane that is clamped all around between the housing of the maintaining chamber and that of the scavenging air chamber. The valve can be used for scavenging one or several filter bodies, e.g. bag filters or sack filters. For years, air filters with this type of cleaning system have been extraordinarily successful because, thanks to the method of construction of the scavenging air valve, they can be driven with a low scavenging pressure. In the case of single bag scavenging, a maximum positive pressure of 0.5 atmospheres suffices. Hence, use of simple air seals is possible.
One disadvantage repeatedly experienced lies in the limitation of the area of application because the maximum operating temperature is limited to about 150.degree. C. by the quality of the rubber membrane.
In another construction, the membrane is not clamped but, rather, freely movable. To close off the scavenging air opening, it clears the line used for filling the scavenging air chamber through the control valve. Therefore, this control valve must be constructed to reverse the direction of flow, and not only for shutting off the exhaust air. Its construction is expensive and its speed of response is much lower. Because the maintaining chamber is not constantly in communication with the scavenging air chamber, scavenging cannot be interrupted simply by closing the control valve at the desired point in time, rather compressed air must be introduced into the maintaining chamber through the control valve so that the scavenging air opening can once again be closed. The speed of response in this case is also lower so that the duration of the scavenging impulse is difficult to control. In order to guarantee a sufficient scavenging effect, this duration of scavenging impulse must be selected to be greater than the first type of construction, such that a considerably greater consumption of scavenging air is necessary. The operating temperature is limited in this case also.
In the case of digital, pneumatic and hydraulic control, there are known valves whose valve body is formed as a fixed disk. They have the same disadvantage as the previously described construction, in that they require a positive control pressure for closure and are not controlled simply by blocking an exhaust air line. Their use is intended primarily for hydraulic applications. Primarily, their purpose is improvement of the sealing effect and reduction in the quantity of oil leakage. Their conversion to the dimensions and to the speed of pneumatic scavenging air valves for air and gas filters is impossible because of the intolerable noise from the striking of the disk against the maintaining chamber housing.